Python Data Structures

📘 What is it

Python has four built-in collection types. Understanding their properties (ordered, changeable, and allowing duplicates) is crucial for technical interviews.

🏗️ Core Structures

List: Ordered, changeable, allows duplicates (e.g., [1, 2, 2]).
Dictionary: Ordered (from Python 3.7+), changeable, no duplicates in keys (e.g., {"key": "value"}).
Tuple: Ordered, unchangeable (immutable), allows duplicates (e.g., (1, 2)).
Set: Unordered, unindexed, no duplicates (e.g., {1, 2}).

⚡ When to use

List: For storing sequences of items where order matters.
Dict: For fast lookups via keys (hashing).
Tuple: For data that shouldn't change (safer memory allocation).
Set: For membership testing and removing duplicates.

🧠 Big-O Summary

Structure	Access	Insert	Remove	Search
List	$O(1)$	$O(n)$*	$O(n)$	$O(n)$
Dict	-	$O(1)$	$O(1)$	$O(1)$
Set	-	$O(1)$	$O(1)$	$O(1)$
*Amortized $O(1)$ for end insertions

💻 Code example


python Standard
# List comprehension
squares = [x**2 for x in range(10)]

# Dict usage
user = {"id": 1, "name": "Alice"}
if "name" in user:
    print(user["name"])

# Set uniqueness
nums = {1, 2, 2, 3} # Result: {1, 2, 3}

❌ Common mistakes

Mutable Defaults: Using a list or dict as a default argument in a function (they are evaluated once at definition time).
List Search: Using a list to check for membership in a loop (use a set for $O(1)$ lookup).
Tuple Mod: Trying to modify a tuple (lead to TypeError).